This invention relates to carbon fibre reinforced plastic (hereinafter abbreviated to CFRP) structural materials comprising light metal, carbon fibre and resin, which are structural materials for use as building materials, or for use in structures, motor vehicles, ships and the like.
Hybrid materials in which a light metal such as aluminium and a carbon fibre reinforced plastic are stuck together have become popular in construction work/building and as structural materials for motor vehicles, ships and the like.
For example, because sections made of aluminium are lighter than sections made of steel, they are used as high-rise building members and the like, but because the elastic modulus of aluminium is low, at about ⅓ that of steel, in cases where section rigidity is important in design terms, it is necessary to enlarge the cross-sectional shape of the aluminium section compared to the case of steel. When the cross-sectional shape is enlarged the amount of material employed is increased and so it is not possible to achieve as much weight reduction as anticipated, and thus the merit of using aluminium sections is reduced. Hence, aluminium sections which are reinforced with carbon fibre reinforced plastic (herein-after abbreviated to CFRP) have been invented (Japanese Examined Patent Publication No. 53-32181).
Again, since motor vehicle bumpers made of aluminium are lighter than those made of steel, they have become popular for the purposes of enhancing motor vehicle fuel consumption and enhancing travel performance, but in the case of an aluminium material the impact behaviour is just simple and within the theoretically-calculated range, so door beams or bumper reinforcing materials in which aluminium and fibre reinforced plastics are combined have been proposed in Japanese Unexamined Patent Publication No. 6-101732 and in Japanese Unexamined Patent Publication No. 4-243643.
However, structural materials for construction work/building, cars, rolling stock and the like are exposed to a high humidity environment at the time of the rainy season for example, so where a light metal and a CFRP have just been stuck together there has been the problem that, during prolonged use, galvanic corrosion occurs due to the natural potential difference between the two, leading to unforeseen damage to the member. Generally speaking, in cases where a light metal and CFRP are stuck together, it has been felt that the thinner the layer of adhesive agent the better (since the adhesive strength increases the thinner it gets, and also so as not to produce a layer of out-flowing adhesive agent when pressure is applied at the time of adhesion). In the inventions described in the aforesaid publications, the concept that it is only by providing a specified thickness of adhesive agent layer that strength, impact characteristics and also environmental resistance are realized, as found in the present invention, is not to be seen.
As a technique for preventing galvanic corrosion between a light metal and CFRP, there is disclosed in Japanese Unexamined Patent Publication No. 61-60772 the bonding of a metal panel and a CFRP with an acrylic adhesive agent containing glass beads of specified diameter. As is clear from the description on page 564 that xe2x80x9cthere are no restrictions with regard to the film thickness conditionsxe2x80x9d, the idea in that publication is the simple way of thinking in circulation that galvanic corrosion can be avoided merely by mixing a glass material into the resin which forms the adhesive agent base (the base resin), but this not a highly reliable technique for avoiding galvanic corrosion. If the insulating property of the base resin in the adhesive agent is inadequate prior to the addition of the glass beads, then no matter what amount of glass beads is added moisture and ionic substances will readily pass through the resin and galvanic corrosion will be produced. Moreover, there has also been the problem that unless, as disclosed in the present invention, the adhesive strength is at a specified level (in said publication, the adhesive strength is 7.6 MPa), galvanic corrosion inevitably proceeds (when exposed outdoors or in a high temperature high humidity environment in which oxygen and moisture are jointly present, rather than in water).
The objective of the present invention lies not just in resolving the aforesaid problem of structural materials comprising a light metal and CFRP so that no galvanic corrosion occurs, but also in providing extremely practical light metal/CFRP hybrid structural materials which are lightweight and outstanding in their rigidity, strength and impact resisting performance.
In order to realise the aforesaid objective, the present invention basically has the following constitution. It comprises light metal/CFRP-made structural members which are characterized in that they are a structural material in which a CFRP material is stuck to the surface of a light metal material via an adhesive agent layer of thickness at least 10 xcexcm and up to 500 xcexcm, and the volume resistivity of the adhesive agent layer between said metal material and said CFRP material is at least 1xc3x971013 xcexa9.cm and, furthermore, the adhesive strength at room temperature is at least 15 MPa.
As stated above, in order to manifest the mechanical properties of a structural member and to prevent galvanic corrosion, it is necessary to ensure that there is a specified thickness of adhesive agent between the light metal and the CFRP. Furthermore, since galvanic corrosion is produced when the moisture absorbed by the adhesive agent layer forms an electrical circuit, it is necessary that the insulation property of the adhesive agent layer itself be at least a certain value when moisture is absorbed. That is to say, the volume resistivity of the adhesive agent itself needs to be at least a specified value. Moreover, it is further preferred that the value of the volume resistivity when moisture has been absorbed is at least a specified level.
Furthermore, what the present inventors have newly discovered is the fact that much of the galvanic corrosion of a structural material is brought about because of an insufficient interfacial adhesive strength. That is to say, if the adhesive strength is below a specified level, then due to the high stresses acting at the interface microscopic cracks are generated within the adhesive agent layer forming pathways for galvanic corrosion, and/or if the adhesive strength is below a specified value then microscopic local separation occurs at the adhesion interface, forming stopping places for the moisture which is the medium of galvanic corrosion, and so galvanic corrosion is brought about. Thus, in order to prevent galvanic corrosion of the structural material, the adhesive strength must be at least a specified level. In other words, by specifying both the insulation property and the adhesive strength, it has become possible for the first time to put to practical effect a highly reliable structural material which is free of galvanic corrosion.
Since, in the art of aforesaid Japanese Unexamined Patent Publication No. 61-60772, the thickness of the adhesive agent layer is not controlled and since the metal sheet and CFRP are not stuck together sufficiently firmly (in Japanese Unexamined Patent Publication No. 61-60772 the adhesive strength is 76 kg/mm2), in a real environment of high temperature and high humidity there is galvanic corrosion of the structural material.